We started on this observation lesson to find out something about the habit of growth of roots, so that we could tell how the roots do their work for the plant.  But before going on with that question, let us stop right here and see whether we cannot find some very important lessons for the farmer and plant grower from what we have already seen.  Is a knowledge of these facts we have learned about roots of any value to the farmer?  Let us examine each case and see.

Of what value is it to the farmer to know that the larger part of the roots of farm plants develop in that part of the soil that has been plowed or spaded?  It tells him that plowing tends to bring about the soil conditions which are favorable to the growth and development of roots.  Therefore, the deeper he plows, the deeper is the body of the soil having conditions best suited for root growth, and the larger will be the crop which grows above the soil.

Of what value is it to the farmer to know that the roots of farm plants penetrate to depths of five or six feet in the soil?  To answer this question it will be necessary for us to know something of the conditions necessary for root growth.  So we will leave this till later.

Of what value is it to the farmer to know that many of the roots of his farm plants come very near the surface of the soil?  It tells him that he should be careful in cultivating his crop to injure as few of these roots as possible.  In some parts of the country, particularly in the South, the tool commonly used for field cultivation is a small plow.  This is run alongside of the row, throwing the soil from the crop, and then again throwing the soil to the crop.  Suppose we investigate, and see how this affects the roots of the crop.

[Illustration:  Fig. 8.  Sweet potato roots.  The great mass of the roots is in the plowed soil.  Many of them reach out 5 to 7 feet from the plant.  Some reach a depth of more than 5 feet, and others come to the very surface of the soil.]

[Illustration:  Fig. 9.  Soy-bean roots showing location, extent and depth of root-growth.]

Let us visit a field where some farmer is working a crop with a plow, or get him to do it, for the sake of the lesson.  We will ask him to stop the plow somewhere opposite a plant, then we will dig a hole a little to one side of the plow and wash away the soil from over the plow (see Fig. 10), and see where the roots are.  We will find that the plow-point runs under many strong-feeding lateral roots and tears them off, thus checking the feeding power of the plant, and consequently checking its growth.  Now, if we can get a cultivator, we will have that run along the row and then wash away the loosened soil.  It will be found that few, if any, of the main lateral roots have been injured.

Is it of any value to the farmer to know that roots extend laterally three to six feet and more on all sides of the plant, and that every part of the upper soil is filled with their branches and rootlets?  This fact has a bearing on the application of manures and fertilizers.  It tells the farmer that when he applies the manure and fertilizers to the soil he should mix the most of them thoroughly all through the soil, placing only a little directly in the row to start the young plant.